In recent years, photonic crystals have been drawing attention as a new optical device. The photonic crystal is an optical functional material with a periodic distribution of refractive index. This structure of the periodic distribution of refractive index forms a band structure for the energy of light or electromagnetic waves. This structure is particularly characterized by the formation of an energy region through which no light or electromagnetic waves can be propagated. (This region is called a “photonic band gap” or “PBG.”) When a disorder (defect) is introduced in this distribution of refractive index, an energy level (defect level) due to the defect is created within the PBG, allowing only light having a wavelength corresponding to the defect level to exist at the defect. By this technique, for example, an optical resonator consisting of a point-like defect or an optical waveguide consisting of a linear defect can be created in the photonic crystal. When the photonic crystal has an optical resonator in the vicinity of an optical waveguide and the resonance wavelength of the optical resonator is within the wavelength range where light can be propagated through the optical waveguide, the photonic crystal functions as an optical multiplexer/demultiplexer that can extract light having a wavelength equal to the resonance frequency from the light propagated in the optical waveguide to the optical resonator (demultiplexing) and merge light having the same wavelength from the optical resonator into the optical waveguide (multiplexing).
Photonic crystals are broadly divided into two-dimensional photonic crystals (for example, refer to Patent Document 1) and three-dimensional photonic crystals (for example, refer to Non-Patent Document 1 and Patent Document 2). The two-dimensional photonic crystal disclosed in Patent Document 1 is a plate-shaped dielectric material in which air holes are periodically arranged. The three-dimensional photonic crystal disclosed in Non-Patent Document 1, which is called “Yablonovite”, is a block-shaped dielectric material having a large number of holes formed in three directions (at 120° intervals), with each hole extending at an angle of 35° from the normal to the block surface. The three-dimensional photonic crystal disclosed in Patent Document 2, which is called a “woodpile” crystal, is a stack of stripe layers, with each layer consisting of dielectric rods arranged periodically and parallel to each other. The stripe layers are stacked so that the rods of any two nearest layers are orthogonal to each other, while the rods of any two next-nearest layers are parallel to each other and displaced by one half of the spatial period thereof. Three-dimensional photonic crystals have the advantage that it barely allows the leakage of light at the defect, thus suppressing the loss of light at the optical resonator or optical waveguide to extremely low levels.
It was conventionally said that the three-dimensional woodpile photonic crystal is difficult to produce since the stripe layers must be accurately positioned relative to each other. To address this problem, one method has been proposed in Patent Document 3. This method creates a three-dimensional photonic crystal by a two-stage etching process; the first etching is performed in a first direction inclined to the surface of a dielectric base body to create a hole extending in the first direction, after which the second etching is performed in a second direction intersecting with the first direction at a predetermined angle to create another hole that extends in the second direction. In this method, the 4nth stripe layer and the next-nearest 4n+2nd stripe layer are created by the first etching (where n is an integer), and the 4n+1st stripe layer and the next-nearest 4n+3rd stripe layer are created by the second etching. This method facilitates the production of the device since it requires no positioning of the stripe layers.
On the other hand, the two-dimensional photonic crystal has the advantage that it is easier to produce than the three-dimensional photonic crystal. In recent years, a two-dimensional photonic crystal that is capable of creating a PBG effective for both TE-polarized and TM-polarized waves (complete PBG) over a wider energy range than in the conventional crystals has been proposed. Patent Document 4 discloses a two-dimensional photonic crystal having a plate-shaped base body with three elongated air holes extending in different directions from each lattice point of a triangular lattice (three-directional oblique holes). The three air holes are formed at angular intervals of 120° when projected on a plane parallel to the surface of the base body, and each air hole is inclined at approximately 36° from the normal to the base body. For a base body made of silicon, this two-dimensional photonic crystal can have a complete PBG with a large width of approximately 15% (which is defined as the ratio of the width of the complete PBG to the energy value at the center of the complete PBG (gap-midgap ratio)). This value is much larger than that of the complete PBG created in conventional photonic crystals, which is no greater than several percents.
Both the three-dimensional photonic crystal described in Patent Document 3 and the two-dimensional photonic crystal having the three-directional oblique holes described in Patent Document 4 can be produced by etching a base body in a direction inclined to the base body by a predetermined angle (oblique etching). The oblique etching can be used not only for the production of photonic crystals but also for the microfabrication of semiconductor devices, the production of microelectromechanical systems (MEMS), and other processes.
Patent Document 5 discloses an etching method in which an electric-field control plate having an edge obliquely formed with respect to the surface of the base body is placed on the surface of the base body, and a bias voltage is applied to the ions in the plasma to make the ions impinge on the surface of the base body. In this method, the equipotential surfaces are deformed along the oblique edge and the ions impinge on the surface of the base body in an oblique direction appropriately perpendicular to the equipotential surfaces, thus achieving the oblique etching.